The present invention relates to human machine interfaces, particularly to those interfaces that measure human body positions and display forces to the human operator.
Teleoperation refers to a mode of enhanced human control of remote manipulators. In teleoperation, the human operator utilizes a controller device or xe2x80x9cmasterxe2x80x9d in combination with his or her body to control the positions of a remote manipulator or xe2x80x9cslavexe2x80x9d device. The slave or remote manipulator may be a physical robotic gripper, or a virtual computer generated representation of a human body part, most commonly a human hand. In teleoperation, relative positions or forces of the human body are measured at the master and used to control the remote manipulator positions or forces at the slave. Most often positions are measured at the master and used to control the slave positions, and forces are measured at the slave to be displayed to the human operator through the master. In virtual slave environments, the forces are calculated within the virtual reality computer, while in robotic slave environments, the forces of contact between the slave and any object are measured via forces sensors. One human body part that is very well suited for teleoperation control is the human hand. The hand is most commonly used for highly dexterous manipulation tasks in interaction with our everyday surroundings. Because of this human ability several instrumented master devices have been developed in order to measure the time varying positions of the fingers on the human hand, utilizing the position data to control the finger positions of a virtual or robotic slave hand. Some of these devices have been developed to also provide a means for display of forces that occur in the slave environment to the human operator in the master environment. However the prior art is lacking in the simplicity necessary to allow large scale acceptance of this method of interaction, and/or are not capable of displaying force sensations that match those of real world interactions, especially at the high rate of speed these sensations are processed by the human operator.
The present invention relates to a master device capable of measurement of finger angular position and display of forces of contact experienced by the slave to the human operator. Both position measurement and force display are performed locally to the fingertips with respect to the hand. Many methods exist for measuring the global position and orientation of the human hand and using this data to control the position and orientation of the slave hand. Some methods include: attaching the master of the present invention to a joystick, attaching the master of the present invention to a six degree of freedom robot, or the use of a magnetic type tracking device.
The research in the area of local finger position measurement and force display dates back at least as far as 1963, when Jones and Thousand (U.S. Pat. No. 3,263,824) disclosed a device for providing the operator with a kinesthetic sensation which simulates the force being placed on a object controlled by the manipulator. This device consisted of an air bladder attached to the interior finger surfaces of an operator worn glove. The bladder is controlled to inflate at a pressure proportional to the force measured at a remote robotic manipulator. One deficiency in this device is that it is difficult to apply and remove the bladder pressure at very high frequencies necessary to approximate real time interaction. A second drawback is that the bladder material gathers and bunches up as the operator moves their finger from the extended position toward the palm to the retracted position, not permitting full finger-retraction.
Previous attempts at hand mounted single controlled degree of freedom finger tip force displays have provided forces that vary widely with finger bend angle, and/or are able to represent motion through only a fraction of the 180 degrees of real finger bend motion. For example, the palm mounted air cylinder method of Burdea""s (1996) Rutger""s Master I and Rutger""s Master II provide a force at an angle that widely varies with finger bend angle, and can only represent finger bend from approximately 40 to 90 degrees. The cylinder mounted to the palm also provides a force to the operator""s palm for all slave/object interactions, even those in which there is no contact between an object and the slave palm area. The angle of applied force also varies with finger bend angle for the tendon approach of Kramer (U.S. Pat. No. 5,184,319). The Kramer device uses tendons that pass along the outer surface of the operator""s hand, thus utilizing the outer surface of the knuckles as a fulcrum point for application of forces to resist finger retraction. With this approach, the tendon force must be very high to restrict finger retraction, and consequently, it is difficult to maintain the tendons in their desired position. Additionally with the Kramer device, the forces applied to the operator""s fingertips are substantially directed along the longitudinal axis of the finger distal digit, when the finger approaches the fully extended and fully retracted positions. In contrast, during real grasp operations, the force of contact between grasped objects and the human fingers are substantially normal (perpendicular) to the longitudinal axis of the finger distal digit. The tendon with additional moment arm modification of Virtual Technologies (1997), more closely provides normal direction fingertip forces throughout full finger bend range, but at the expense of applying ghost forces to the back of the second finger phalange.
Other approaches to finger force display masters employ devices that do not permit a large workspace for hand motion, attach heavy actuators to the human body, provide forces at widely varying angles with respect to the longitudinal axis of the distal finger digit, limit finger bend position to only a small portion of its full range, or are very expensive to manufacture. Additionally prior approaches fail to provide a force display signal to the operator at a high enough rate of speed to simulate real world touch sensations, thus resulting in jerky or lively object sensations, unstable oscillatory force applications, or require very slow human command motions.
It is an object of the invention to provide a means of displaying forces substantially normal to the fingertip through out the full range of finger bend, for use in robotic and virtual reality environments, without the aforementioned shortcomings. The application of forces will permit simulation of grasping tasks, touching surfaces with the fingers extended, deforming virtual object shapes and simulating the removal of material from a virtual body such as virtual sculpting.
It is an object of the present invention to provide a master device which is of simple construction and requires the minimum number of controlled degrees of freedom to accurately measure fingertip positions and provide force display thereto, and thus require the minimum number of measured control variables and the minimum number of actuators.
It is an object of the present invention to provide a device for force display that without modification can be effectively used by people with a wide range of hand sizes.
It is also an object of the invention to provide to provide a master device for teleoperation that exhibits a high degree of transparency to the operator, such that unobstructed motion of the slave, is represented to the operator as free unobstructed motion of the fingers throughout a large range of finger motion.
It is another object of the invention to provide a master device that is portable and thus provides the operator with a large workspace.
It is a further object of the invention to provide a master device that is comfortable to wear for long periods of time and thus is light weight, has low apparent (to the user) inertia, and yet maintains a high stiffness so as to provide realistic rendering of forces encountered when grasping rigid objects.
Yet another object of the invention is to provide a master device which is of simple construction such that manufacturing costs will be reasonable and that said device may enjoy wide acceptance in the marketplace.
Another object of the present invention is to provide a master device that is capable of representing initial contact with remote objects at a high rate of speed, permitting teleoperation that is stable and may be done in normal human speed, and provides user force display that closely resembles forces experienced in real human hand and object interaction.
Another object of the invention is to provide a master device that is very safe to operate and permits the operator to quickly and easily disable all force display to their body.
Still another object of the invention is to provide a master device that provides minimal xe2x80x9cghosfxe2x80x9d forces (or forces that in real manipulation are not present) to the operator throughout all modes of operation. These and other advantages of the present invention will appear from the following description, accompanying drawing and appended claims.
In accordance with one embodiment of the invention, a master device is provided having at least one linkage assembly mounted to the operator""s body with a base connection to the back of the users hand and a distal link that connects to the user""s fingertip. Between the distal connection and the base two pivotal links are provide in series such that the first link is pivotally connected to the base and the second link is pivotally connected to both the first link and the distal link. The links are configured such that rotation of the first link with respect to the base provides motion of the distal that may follow the normal motion of finger bend during grasp tasks. The two links are further configured such that throughout the full bend range of motion for the finger, the second link remains approximately perpendicular to the distal link and thus the fingertip pad.
A sheathed cable transmits the rotational motion of the first link, with respect to the base, wherein tension applied to the cable results in relative motion between the cable and the sheath. In the present invention a proximal end of the sheath is mounted to the base such that longitudinal motion of the sheath with respect to the base is prohibited. The proximal end of the cable is mounted to the first link such that longitudinal motion of the cable with respect to the first link is prohibited. As such rotation of the first link with respect to the base (and thus the finger bend) is reflected by longitudinal motion of the cable with respect to the sheath. This is advantageous as the motion permits mounting of the position measurement and force providing assembly in a location remote from the users hand. Thus the operator need not carry the weight and bulk of the aforementioned assemblies.
It will be appreciated that while each finger has three degrees of freedom along the longitudinal plane, the degrees of freedom are usually not controlled independently by human beings in grasping tasks. The finger linkage is designed so that while three total degrees of freedom were present, finger bend toward and away from the palm could be represented by a single degree of freedom. This one degree of freedom is to be used for angular position measurement of the finger, and for force display, resisting finger bend toward the palm. By representing finger motion and restriction thereof with one degree of freedom in the above described the finger bend linkage, both the mechanical structure and the computation requirements of force display control are greatly simplified, and thus approach fulfillment of the objectives of high-speed response, and low system cost. Additionally because there are two uncontrolled degrees of freedom, the linkage is effective for a variety of operator""s hand sizes, without the need for adjusting link lengths. Because the finger linkage only contacts the human body at the fingertip and main body of hand, display of forces can only be felt at these locations. This is advantageous because the back of hand surface (opposite the palm) is one area of the body least sensitive to force application, and the fingertip is the desired force display target. Thus, xe2x80x9cghostxe2x80x9d forces, or forces applied to the operator that would not normally exist in direct real manipulation are kept to a minimum. Many prior approaches to force display have provided ghost forces in a variety of locations, often at relatively high magnitudes. Another benefit of the linkage design is that throughout 180 degrees of motion for the finger, resistance forces are applied substantially perpendicular to the distal finger pad, accurately representing the direction of normal forces that occur during real grasping and manipulation of objects.
To fulfill the objective of low resistance to motion while bending fingers in the absence of virtual object collision, a remote xe2x80x9creplicated fingerxe2x80x9d pivotal bar is attached to the distal end (or remotely mounted end) of the cable. The replicated finger duplicates the motion of the operator""s finger, as a single degree of freedom pivotal bar, scaled by a factor of two (approximately). Thus, as the operator""s finger moves from the 0 to 180 degree bend positions, the replicated finger moves from a 0 to 90 degree position. Since the replicated finger provides a scaled replicate of the real finger motion, the position of the replicate finger is used to represent the position of the slave finger. Additionally, resistance to motion of the replicated finger results in resistance to motion of the operator""s finger. Taking advantage of this, a de-coupled contact drum is pivotally mounted in the plane of the replicated finger, such that the drum may be positioned to contact the replicated finger at any degree of rotation. This configuration allows a computational advantage for grasp tasks because when grasping an object, people must first move their hand near the object with the fingers forming a pre-grasp shape larger than the object to be grasped, and then bend their fingers toward the palm to grasp the object. Thus, it is possible to move the contact drum to a position such that it will interfere with finger bend while in the pre-grasp formation, before the finger has begun to bend inward to grasp a virtual object, resulting in better system response time for grasp tasks. The contact drum is positioned by a DC gear motor, under position control. This arrangement results in a first condition of contact or non-contact of the contact drum with the replicated finger, and a second condition of variable force between the contact drum and the replicated finger, which is controlled via pulse width modulation (PWM) by a master control computer. An advantage of the de-coupled actuator arrangement compared to directly coupled actuators (employed by prior master interfaces) is that direct-coupled actuators provide a force only after virtual finger/virtual object interference, which may appear xe2x80x9cjerkyxe2x80x9d if very high control speed is not maintained. Another advantage is that the actuator need not be driven in contact with the user or be freely back driven, during free motion in order to appear transparent to the user, and thus permits higher transparency with a lower cost motor. Additionally, the de-coupled actuator provides advantages of increased stability during grasp tasks (since the contact drum is essentially stationary), and the ability to present high stiffness virtual objects.